If you go to VM, and click on Tax->Add Tax Rate you will be surprised that the tax won't update on the checkout page.
I used 0.09 as tax rate for California-US. If I change the state on billing address and the "default (Same as billing)" option is selected in Shipping Address, nothing will happen.
Only if you click on the "New Address" option and you fill the fields, the tax rate will be updated.